There have hitherto been known certain hydraulic circuit arrangements for a hydraulic press in which a slide is vertically driven up and down by a hydraulic cylinder assembly, as disclosed, for example, in Japanese Examined Utility Model Publication No. Hei 2-18801, Japanese Unexamined Utility Model Publication No. Hei 6-39285 and Japanese Unexamined Patent Publication No. Hei 6-155089.
In particular, Japanese Examined Utility Model Publication No. Hei 2-18801 has described a fluid pressure control circuit arrangement having a construction in which a circuit for supplying a pressure fluid into a hydraulic cylinder assembly is provided with a directional control valve and a pilot check valve such that by switching the directional control valve so as to allow the fluid pressure to be applied via the check valve to the hydraulic cylinder assembly, the latter may act to drive any particular load operatively associated therewith.
Also, Japanese Unexamined Utility Model Publication No. Hei 6-39285 has specifically disclosed a hydraulic circuit arrangement for a press in which the cylinder assembly is constructed to include a pressure cylinder unit having a larger pressure receiving area and a subsidiary cylinder unit having a smaller pressure receiving area but operable at a higher speed, the two cylinder units being arranged on a common center line. And, there a construction is employed in which the respective pistons of the two cylinder units are coupled together by a piston rod in the two rod cylinder assembly with the piston rod in the high speed cylinder unit projecting upwards thereof.
And, the system is so configured that after the high speed cylinder unit is supplied with the pressure fluid so as to allow its piston to be driven at a higher speed, the pressure cylinder unit may be supplied with the pressure fluid to allow a larger pressing force to be derived so as to be capable of meeting with a greater load.
Further, a specific hydraulic system as disclosed in Japanese Unexamined Patent Publication No. Hei 6-155089 has also adopted a cylinder assembly constituted of a high speed cylinder unit and a pressure cylinder unit. In this specific arrangement there is provided at the side of the piston in the pressure cylinder unit a sequence valve that is adapted to be opened and closed by using a pilot pressure.
And, the system specifically disclosed there is so configured that by permitting the sequence valve to be switched on and off, a high speed operation may be switched to a pressing operation, thus being capable of meeting with a greater load at a higher speed without requiring a special piping unit or valves externally fitted.
With a fluid pressure control circuit arrangement as disclosed in Japanese Examined Utility Model Publication No. Hei 2-18801 it is noted, however, that where a dust is introduced onto a spool in the directional control valve so as to disturb its downward displacement the pressure fluid from the pressure cylinder may no longer be constricted, thus bringing about the inconvenience that there can arise the danger of an increased load.
With a hydraulic circuit arrangement as disclosed in Japanese Unexamined Utility Model Publication No. Hei 6-39285, it is also noted that a bite into a die may arise during a pressing operation while a large force of removal may not be obtained. There then arises an inconvenience that a pressed sheet may not be removed from its die bite.
It further is noted that with a high speed for a greater load cylinder assembly as disclosed in Japanese Unexamined Patent Publication No. Hei 6-155089 in which the sequence valve is provided internally of the piston in the pressure cylinder unit, not only is its maintainability poor but also there may be brought about a danger arising from the fact that the piston rod in the high speed cylinder unit is projecting upwards thereof.
With the above described problems taken into account it is an object of the present invention to provide a high speed safety circuit arrangement for a hydraulic press that is capable of permitting a slide to be driven and operated at an increased speed and yet with due safety.